I have a HK870 power amp which I purchased new about 32 years ago.Physically the amp is in excellent condition and I currently use it to power a second zone in my home. It just doesn't sound as open or clear as it once did. A year or so ago, I replaced the input coupling caps, thinking that would help, but I honesty cant tell a difference. I concede, it could be my hearing, but given it's age, it could be the electrolytic caps, and or other components / adjustments. I'm looking for opinions on if it's worth recapping and restoring, or if I'm better off putting the money towards a new, or gently used amp. Thanks in advance for your thoughts.
Harman Kardon Recap
- Thread starter Prudent
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- amplifiers harman kardon hk hk870 recap
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Assuming that the sound degradation is indeed the amp itself, and not any other component in your system, a recap may be worthwhile. Looking at the schematic, there aren't that many electrolytics used, so the cost of a full re-cap shouldn't be much. I would go for it. Given the age it's not an unreasonable hypothesis that the caps are not what they once were. You now have an opportunity to use better-than-original caps, so potentially the result will be better than new.
Looks like our HK guru @dlucy may be performing the same service - see https://audiokarma.org/forums/index.php?threads/harman-kardon-hk870-power-amp-restoration.789670/ - he'll likely be able to offer more specific / valuable advice than me.
Looks like our HK guru @dlucy may be performing the same service - see https://audiokarma.org/forums/index.php?threads/harman-kardon-hk870-power-amp-restoration.789670/ - he'll likely be able to offer more specific / valuable advice than me.
The caps are easy to get to and there are few of them. I forget if I’ve run up a spreadsheet in replacements yet. I did all the little caps and diodes, but not the big ones yet. Last week I finally put it back together (power switch problems) and it works week, functionally. I don’t know if mine, with original large caps and new small caps, sounds any different as I haven’t listened critically yet.
The amp design is very good. It is a great era for HK and this was close to the TOTL.
The amp design is very good. It is a great era for HK and this was close to the TOTL.
For power path caps I use Nichicon PW series, so they are not posh really, they are just the perfectly correct operational caps (lowest ESR and highest ripple current).
I’ll assemble what I’ll use and post it tonight.
Thanks for the feedback - pun intended. Local repair shops won't provide a quote or any cost guidance without seeing the amp and a $60 to $70 fee, which would be applied to any service work. Looking at caps on Mouser, all the electrolytic caps will cost about $125. Applying shop mark-up on parts, and adding a few hours for labor, I estimate the recap will cost about $350. I have a basic knowledge of electronics, and digital VOM and some soldering skills. I am thinking about attempting the recap myself. I know I have to discharge the large power supply caps before touching or removing them, and get the polarity of the replacement caps correct. I'd use low ESR, high ripple value caps for the pre-driver and driver power supply, Nichicon, Rubycon, or the like, and Audio Grade caps from Nichicon or ELNA for the handful of smaller value electrolytic. Thoughts and suggestions welcome.
fred soop
Super Member
I have recapped 2 Citation 21 preamps. One with Panasonic, the other with Nichicon. It seems that HK had excellent designs in that era but they were going cheap on capacitors. Many of the small electrolytics had voltage ratings too close to the operating voltage. If you have a schematic, check or calculate the voltages on these capacitors and make sure the voltage ratings of the new caps are sufficiently high. The new parts will be smaller, so if a higher voltage is larger, it will probably still fit. The main power supply caps are probably ok.
On the Citation 21 units, a single cap or two could be replaced by only removing the bottom cover, but with a full recap, it was easier to disassemble and remove the main circuit board.
While you have it open, clean the switches. This seems to be another problem area.
On the Citation 21 units, a single cap or two could be replaced by only removing the bottom cover, but with a full recap, it was easier to disassemble and remove the main circuit board.
While you have it open, clean the switches. This seems to be another problem area.
The hk870 is not hard to work on. The only thing I see as daunting is the terminal style of the replacement caps. The originals are leaded, so those leads just push through the through holes in the PCB. If you end up having to choose a snap-in style terminator instead, due to what capacitance values are available, you may have to drill some holes in the PCB. Are you up for that? Shouldn't be difficult.
My personal preference for power path electrolytic caps is Nichicon PW series as they are low ESR and high ripple current. If they don't have something in that family that will work for me, I look for a snap-in from Nichicon.
In this case, searching for capacitors, aluminum electrolytic, in stock, 10,000 to 12,000 (one step up) uF, with 63 or 80 (one notch up) VDC yields only a few choices... and all snap-in, sorted here by price each, ascending:
The LKG datasheet explains what the higher-cost, last-letter-indicating differences are. For me, this is marketing hype, but each to their own. I would choose the lower cost 80 VDC one to feel comfortable about running the cap at less than its rating all the time.
You will have to drill the PCB to use these above.
For the smaller power supply caps, searching for capacitors, aluminum electrolytic, in stock, 4,700 to 5,600 (one step up) uF, with 63 or 80 (one notch up) VDC yields more choices and they are all U-something series, so they are leaded (no drilling required unless the lead spacing is off). There are UVK, UVR, UVY, and UVZ (which are all fine) and UFW and UKW choices. My personal preference is the latter two, but any of these will work. the two I like would be:
For audio path electrolytic caps, I put in the low-leakage Nichicon KL series, but nothing stops you from choosing from the many more-exotic caps.
My personal preference for power path electrolytic caps is Nichicon PW series as they are low ESR and high ripple current. If they don't have something in that family that will work for me, I look for a snap-in from Nichicon.
In this case, searching for capacitors, aluminum electrolytic, in stock, 10,000 to 12,000 (one step up) uF, with 63 or 80 (one notch up) VDC yields only a few choices... and all snap-in, sorted here by price each, ascending:
- 10,000 uF 63 VDC (L series for snap-in) KG 647-LKG1J103MKN
- 10,000 uF 80 VDC (L series for snap-in) KG 647-LKG1K103MKNF
- 10,000 uF 80 VDC (L series for snap-in) KG 647-LKG1K103MKZ
The LKG datasheet explains what the higher-cost, last-letter-indicating differences are. For me, this is marketing hype, but each to their own. I would choose the lower cost 80 VDC one to feel comfortable about running the cap at less than its rating all the time.
You will have to drill the PCB to use these above.
For the smaller power supply caps, searching for capacitors, aluminum electrolytic, in stock, 4,700 to 5,600 (one step up) uF, with 63 or 80 (one notch up) VDC yields more choices and they are all U-something series, so they are leaded (no drilling required unless the lead spacing is off). There are UVK, UVR, UVY, and UVZ (which are all fine) and UFW and UKW choices. My personal preference is the latter two, but any of these will work. the two I like would be:
- 4,700 uF 63 VDC 647-UKW1J472MRD
- 4,700 uF 63 VDC 647-UFW1J472MRD
For audio path electrolytic caps, I put in the low-leakage Nichicon KL series, but nothing stops you from choosing from the many more-exotic caps.
+1
The general rule of thumb is you want caps that are only 60-70% of max voltage under normal operation. You may see this indicated on vintage caps, e.g. "50 WVDC 63 VDC" for 50 VDC working and 63 VDC max.
Was thinking about up-sizing the PS cap values one size to 12,000 and 5,600 respectively, but hadn't thought about increasing the voltage rating. This is a good idea, and the added cost in nominal. Time to do a little shopping and give it a try - not much to lose. If it works, I will have an amp that will outlast me, and if it doesn't work, I will have learned more about electronics.
Excellent plan... rock on!
Reliving.
Active Member
You,be been very helpful here with Prudent and Me .....
on most Forums I have been on ... they start off helpful , then the backbiting starts and then the Forum goes dead.... i’m glad it’s not the case at AudioKarma....
So as a newbie .... just to make it clear you can up the Voltage of a capacitor say from 50 volts to 63 volts so cap has some battery space so to speak.....can you do this method for the capacitance .
Cheers Matt from the UK.
Thanks for the kind words. AK, in general, has helped me greatly in the past few years and I'm happy to pay that back now and again. Do keep in mind that I'm only a hobbyist, not a professional technician or E.E., so be sure to consider all the input you get and not just mine.
Yes, you can increase the voltage value of a replacement capacitor without changing its performance. The only difference is the higher-voltage cap will (usually) be physically larger than a same-voltage cap. The same is true for resistors and wattage, transformers and amperage, diodes and peak reverse voltage, transistors and most of their primary specs.... hmmm... that may be all i can think of.
Redcatcher199th
FREEDOM Isnt FREE TYVETS
I have one I scored from a thrift store, I have not powered it up in 15 year's. I hope you get it figured out.
it's time for me let go of some of my stuff I can say i am a hoarder 
it's time for me let go of some of my stuff I can say i am a hoarder